Interrogators and non-contact transponders are known, and one such system is disclosed in European Patent Application Publication No. 0 585 132, CSIR. In that application, a transponder is identified by synchronising an extracted data signal with a clock signal, but such an interrogator cannot detect small frequency changes in a system having a broad, flat Q factor. In EP 0 374 018 Etat Francais, a smart card detector incorporates a Colpitts oscillator of variable frequency, but the frequency variation is substantial and is dependent on two capacitors and the inductance when the detector is coupled to a smart card. Such a detector cannot detect small frequency changes.